As audio devices, speaker arrays and speaker matrices are fast becoming increasingly popular. The speaker arrays and the speaker matrices are capable of bringing three-dimensional audio (3D audio) to a listener and play a very important role in the 3D entertainment. The speaker arrays and the speaker matrices enable the creation of novel aural sensations, such as virtual sources in front of or behind the speaker arrays and the speaker matrices, by the principles of wave field synthesis, to realize a wide sweet spot (the most appropriate listening position) and a wide stereo sensation. It is to be noted that in the following description, a speaker array is explained as an example, and since the same will apply to a speaker matrix, explanations thereof are merely omitted. In other words, the following explanations about a speaker array imply explanations about a speaker matrix.
Two main known principles of the wave field synthesis are a Rayleigh integral method and a beamforming method. FIG. 1A shows the principle of Rayleigh integral wave field synthesis, and FIG. 1B shows the principle of beamforming wave field synthesis.
The Rayleigh integral is used in the synthesis for a virtual source (a primary source 11) that is present behind a speaker array 10A as shown in FIG. 1A.
The use of the Rayleigh integral allows the wavefront from the primary source to be approximated by a distribution of secondary sources. To put it simply, the primary source 11 refers to the virtual source intended to be synthesized behind the speaker array 10A, and the secondary sources refer to the speaker array 10A itself as shown in FIG. 1A.
Thus, the Rayleigh integral wave field synthesis can be achieved by emulating the amplitudes and delays of the wavefront of the primary source 11 (the virtual source) arriving at each of the secondary sources (the speaker array 10A).
The beamforming is used in the synthesis for a virtual source 12 that is in front of a speaker array 10B as shown in FIG. 1B.
According to the principle of beamforming wave field synthesis, delays and gains are applied to an audio signal outputted from each channel of the speaker array 10B so that as much audio as possible overlaps at a desired virtual spot, which allows the virtual source 12 to be generated in front of the speaker array 10B as a result of the synthesis.
However, existing content is primarily reproduced using stereo sound sources.
For this reason, technologies that enable monaural sound sources or stereo sound sources with speaker arrays to generate novel aural sensation are being actively developed.
For example, Patent Literatures (PTL) 1 to 10 disclose technologies that widen stereo sound images on speaker arrays sing reverberation.